1. Field of the Invention
This invention relates to epoxy resin compositions. More particularly, the invention relates to epoxy resin compositions which are curable at ambient temperatures to afford products having useful properties at high temperatures.
2. Description of the Prior Art
Epoxy resins are among the most versatile of the plastic materials. Because of their toughness, adhesion, chemical resistance and electrical properties, the combination of which is not found in any other single organic polymeric material, the epoxy resins are widely used in coating, adhesives, casting, molding, laminating, potting and encapsulation, and reinforced plastic applications. In general, the epoxy resin is not used by itself but requires the addition of a curing agent or hardener to convert the resin to a crosslinked material. Curing agents which are commonly employed with epoxy resins include aliphatic and aromatic amines, polyamides, tertiary amines, amine adducts, acids, acid monoanhydrides, acid dianhydrides, aldehyde condensation products, and Lewis acid type catalysts. Selection of an appropriate curing agent depends upon system requirements such as mixture viscosity, system mass and temperature, and the characteristics desired in the cured resin such as resistance to temperature and chemicals, electrical properties, and the like.
In recent years, there has been an increasing demand from the aerospace industry and other industrial applications for materials having high-temperature utility. High-temperature utility can be improved through the use of anhydride and certain amine curing agents at elevated curing cycles, as well as through the use of epoxy resins obtained by the epoxidation, with peroxy compounds, of double bonds in certain Diels-Alder adducts. However, in many applications the high-temperature utility is insufficient. Studies indicate that temperature resistance, as well as chemical and heat resistance, is a function of crosslink density of the cured resin, with higher crosslink density affording improvements in these properties. Higher crosslink density can be achieved by increasing the functionality of either the epoxy resin or the hardening agent.
Organic acid dianhydrides which contain other cyclic or aromatic structures and have high functionality have been found to impart improved high temperature resistance as well as increased chemical and solvent resistance, to cured epoxy resin compositions. Illustrative of such acid dianhydrides are pyromellitic dianhydride, cyclopentadiene dianhydride, and benzophenone tetracarboxylic dianhydrides. Unfortunately, these organic acid dianhydrides suffer from the drawback that they are generally high melting solids which are not soluble to any appreciable extent in common solvents or epoxy resins and therefore are difficult to incorporate in epoxy resins except at elevated temperatures. In addition, epoxy resin compositions containing these organic acid dianhydrides as curing agents require high temperature curing cycles.
Thus, there is a continuing search for new epoxy resins and curing agents which can be cured at low temperatures to afford cured resin systems having good high temperature stability.